In the following vocabulary based sentence completion question, choose the correct word to fill the blank: There is now a reasonably ______ body of evidence suggesting that, even from a rigorous scientific perspective, unselfishness and concern for others are in our interests and are, in some sense, innate to human nature.

Introduction / Context:
This question deals with the correct descriptive adjective for a "body of evidence". The sentence claims that there is strong support for the idea that unselfishness is both beneficial and natural from a scientific perspective. You must choose a word that accurately describes the size or weight of the evidence in a positive way.

Given Data / Assumptions:

• The phrase around the blank is "a reasonably ______ body of evidence".
• Evidence here supports a serious claim about human nature and unselfishness.
• The options are "substantial", "small", "insignificant", and "ethereal".
• The adjective must be consistent with the idea that this evidence is persuasive and worth noticing.

Concept / Approach:
In academic and scientific writing, "a substantial body of evidence" is a standard phrase that means a significant amount of reliable data or research. The word "small" would weaken the claim and conflict with the phrase "reasonably", which suggests enough evidence to be convincing. "Insignificant" means unimportant, which again contradicts the message of the sentence. "Ethereal" means delicate, light, or not solid, which does not fit the idea of solid scientific evidence. Only "substantial" accurately expresses that the evidence is strong and considerable.

Step-by-Step Solution:
Identify the tone: the sentence aims to show that science supports unselfishness and concern for others.Recognise that a positive claim requires a reasonably strong amount of evidence, not a small or unimportant amount.Recall common collocations such as "substantial body of evidence" used in scientific literature.Reject "small" and "insignificant" because they carry negative or weakening meanings.Reject "ethereal" because it describes something vague and insubstantial, the opposite of what we want for evidence.

Verification / Alternative check:
Insert "substantial" into the sentence: "There is now a reasonably substantial body of evidence suggesting that, even from the most rigorous scientific perspective, unselfishness and concern for others are not only in our own interests but also, in a sense, innate to our biological nature." This version reads smoothly and matches the formal tone of the passage. The other options produce awkward or contradictory phrases such as "a reasonably small body of evidence" or "a reasonably ethereal body of evidence", which do not fit the context.

Why Other Options Are Wrong:
Option B, "small", contradicts the idea that there is enough evidence to make a strong claim. Option C, "insignificant", suggests that the evidence does not matter, which is the opposite of the intended message. Option D, "ethereal", is used for things that are airy, delicate, or almost unreal, and does not work with the concrete noun "body of evidence".

Common Pitfalls:
Sometimes exam candidates focus only on the word following "reasonably" and assume that anything sounding formal might fit. However, context is crucial. When you see "body of evidence" in an academic style sentence, "substantial" should immediately come to mind as the natural descriptive adjective. Building familiarity with such common academic phrases improves both comprehension and exam performance.

Final Answer:
The correct completion is substantial, giving the phrase "a reasonably substantial body of evidence".